This invention relates, in general, to interferometric characterization of optical elements and is specifically directed to an interferometric thickness test apparatus and method for measuring the thickness variations of optical parts, such as, IR windows, corrector plates, domes and conformal windows.
A variation in thickness of these IR optical parts produce a wavefront distortion which is potentially very detrimental to the performance of the systems in which they are used, so testing for thickness variations of an IR optical part and producing a map showing locations of any thickness variations is highly desirable.
Commercially available mid-wave IR interferometers (10.6 micron wavelength) have a limited ability to measure thickness variations in such IR optical parts. They require precise positioning relative to the surface of the optical part and its radius of curvature. If the optic being tested is in the mid or near-IR wavelength, or the transmitted wavefront accuracy requirement is small, their inherent insensitivity reduces their usefulness for thickness uniformity testing. Hence, there is a need for making a direct, non contact, measurement of the thickness uniformity of optical parts without the need of any critical alignment of the test apparatus. There is also a need for providing a map or profile of any variations in thickness in the optical parts.